Add new settings handling; add new plugin routes; update client

src/main/resources/settings/settings.properties

src/main/scala/de/tobias/playpad/server/PlayPadServer.scala

src/main/scala/de/tobias/playpad/server/plugin/Plugin.scala

...la/de/tobias/playpad/server/server/plugin/PluginGet.scala

...a/de/tobias/playpad/server/server/plugin/PluginList.scala

...s/playpad/server/settings/PropertiesSettingsHandler.scala

...in/scala/de/tobias/playpad/server/settings/Settings.scala

...a/de/tobias/playpad/server/settings/SettingsHandler.scala

...la/de/tobias/playpad/server/settings/SettingsLoader.scala

...ala/de/tobias/playpad/server/settings/SettingsSaver.scala

...e/tobias/playpad/server/transformer/JsonTransformer.scala

src/test/java/ClientTest.java